Knife sharpener



FIG. 1 is a front perspective view of a knife sharpener of our new design;

FIG. 2 is a back perspective view thereof;

FIG. 3 is a top view thereof;

FIG. 4 is a bottom view thereof;

FIG. 5 is one end view thereof;

FIG. 6 is the opposite end view; and,

FIG. 7 is similar to FIG. 1, showing a variation of the design. 

The ornamental design for a knife sharpener, as shown and described. 